This invention relates to a system and method for production of high purity argon from a crude argon feed stream received from a cryogenic rectification plant. This invention also relates to a system and method for improving argon recovery from an air feed and a process which is efficient and lower in cost than prior art systems and methods.
For many years the classical method employed for the refining of argon included a low ratio column to produce crude argon and to remove the bulk of the oxygen, followed by catalytic deoxidation with hydrogen to remove the final traces of oxygen, and finally a high ratio column to remove nitrogen and the excess hydrogen. Availability of hydrogen and its accompanying high cost made this method a less than desirable technique. More recently the use of superstaged packed columns made it possible to bring the oxygen and nitrogen impurities down to specifications by distillation without the need for catalytic deoxidation with hydrogen.
Nguyen, et al., in U.S. Pat. No. 5,730,003, describe a system for producing high purity argon in which crude argon produced in a cryogenic rectification plant is processed in a pressure swing adsorption unit. The argon-rich stream, at about 13 per cent argon, withdrawn from the ultrastaged upper column contains only about 0.4 ppm nitrogen and about 87 per cent oxygen. Processing this stream in the low ratio column brings the feed to the vacuum pressure swing adsorption (VPSA) system to 98.5 per cent argon, 3 ppm nitrogen and 1.5 per cent oxygen. The VPSA system then removes the oxygen in the argon stream resulting in an oxygen impurity less than 1 ppm and a nitrogen impurity less than 3 ppm. Residual gas from the pressure swing adsorption unit is recycled to the cryogenic rectification plant for recovery of its argon content. High purity argon is cooled prior to recovery against oxygen-containing fluid and is subsequently passed into the cryogenic rectification plant. The integrated VPSA process produces high specification argon product at high recovery and low equipment cost. No high ratio column is employed.
U.S. Pat. No. 5,557,951 of Prasad et al. relates to a method of producing high purity argon using a cryogenic argon column in combination with a solid electrolyte ionic or mixed conductor membrane.
A cryogenic air separation system which improves argon recovery is disclosed by Howard et al. in U.S. Pat. No. 5,469,710, wherein vapor from the argon column top condenser is turboexpanded to generate refrigeration and is then passed into the upper column. No adsorption system is used in the purification process.
U.S. Pat. No. 5,313,800 to Howard et al. describes a process for maximizing the recovery of argon from a double column air separation system having a side arm column for argon recovery. A compositional measurement is made at one or more stages of rectification which exhibit high sensitivity to plant process variations. Total nitrogen content in the argon feed may then be computed by simulated mathematical correlation from such compositional measurement.
U.S. Pat. No. Re 34595 of Chen and Cook describes a process for purifying argon from a cryogenic air separation plant. The argon stream is heated and compressed and then permeated through a solid electrolyte membrane selective to the permeation of oxygen. The purified argon can then be distilled to remove nitrogen.
Bianchi, Bonaquist and Victor disclose a cryogenic rectification method in U.S. Pat. No. 5,133,790 over a defined height within the upper column of a double column system to reduce the nitrogen concentration of an argon, nitrogen, and oxygen mixture while maintaining the argon concentration at or near its maximum, thereby enabling production of nitrogen-free argon directly from the argon column system.
Argon is recovered in U.S. Pat. No. 4,477,265 of Kumar et al. from a gas stream by a vacuum swing adsorption process wherein the gas is passed through an adsorbent bed having thermodynamic selectivity for adsorption of nitrogen and the unadsorbed portion then passed through a second adsorbent bed having kinetic selectivity for retaining oxygen. Both beds are regenerated by vacuum desorption. Further purification of the recovered argon may be carried out, if desired, by catalytic hydrogenation of the residual oxygen. The waste gas from the VSA unit may be recycled to the main column of the cryogenic air separation unit.
It is therefore an object of this invention to provide a system and process for producing product argon with 99.9999 mol per cent argon with less than 1 ppm oxygen and less than 1 ppm nitrogen. Improvements in the refining of crude argon in a cryogenic air separation system have long been sought. Economic factors along with more stringent purity specifications for argon increase the need for improved processing to more completely eliminate both oxygen and nitrogen from the argon product.
The argon concentration in the air to be processed in an air separation plant is 0.93 per cent. Argon is concentrated near the middle of the upper column of a cryogenic plant to about 13 per cent with 87 per cent oxygen and 550 ppm nitrogen. This argon-rich stream is processed in a low ratio column to produce crude argon by raising the argon concentration to 97.5 per cent with 1 per cent nitrogen and 1.5 per cent oxygen. An objective of this invention is to provide a novel efficient and low cost method and system of removing the remaining concentration of oxygen and nitrogen from the crude argon stream without the need for ultrastaged columns and without requiring dual VPSA systems for oxygen and nitrogen removal.
The method and system of this invention utilizes one or two argon cryogenic distillation columns and a single VPSA system for oxygen removal from the effluent from a low ratio argon column. Nitrogen is eliminated from the crude argon feed stream by utilizing a high ratio column for this purpose. According to the system of this invention, residual gas from the VPSA unit can be recycled back to a cryogenic plant for processing of the argon content in the residual gas thereby providing higher argon recovery. The novel integration of the VPSA system with the cryogenic column system with its unique recycle of argon-rich xe2x80x9cresidualxe2x80x9d gas provides a higher over-all argon recovery than the prior art (92 per cent versus 82 per cent). This coupled with an accompanying reduction in capital cost makes this method and system of this invention a decided improvement over existing technology.